The present invention relates to a radially anisotropic sintered R-Fe-B-based magnet (R is at least one rare earth element including Y) for use in various application field such as motors, sensors, etc., and a production method thereof.
In the known methods of producing a sintered R-Fe-B-based magnet, a die having an axial length (length along the axial direction or the compacting direction) corresponding to the axial length (hereinafter referred to as "L") of a magnet to be produced. Therefore, a die having a large size in the compacting direction is required when a magnet having a large L. A large size of the die causes several problems such as a difficult handling of the die when mounting to or removing from the compacting apparatus, a large size of the compacting apparatus due to an excessively large compacting stroke.
The radially anisotropic sintered R-Fe-B-based magnet (hereinafter referred to as "R.R. magnet") has been conventionally produced by a compacting apparatus which has a die constituting a magnetic circuit. An example for such a compacting apparatus is shown in FIG. 1. A cylindrical die 9 basically consists of a ferromagnetic portion 1, a non-magnetic portion 2 surrounded by a lower coil 7, and a core 3 made of a ferromagnetic material. A starting powder is charged into a cavity 10 defined by the outer peripheral surface of the core 3, the inner surface of the ferromagnetic portion 1 and the upper surface of a lower cylindrical punch 5 which is movable downward and upward along the axial direction. Then, an upper punch 4 surrounded by an upper coil 6, which is movable downward and upward along the axial direction, moves downward into the cavity 10 to compact the starting powder to produce a green body. The green body is then sintered to produce an R.R. magnet.
The intensity of the orientation magnetic field (Bg) applied to the cavity 10 is expressed by the following formula (1): EQU Bg=d.sup.2 .times..sigma..sub.s /(4.times.D.times.Lm) (1)
wherein d is an outer diameter of the core 3, D is an inner diameter of the die 9, Lm is a length of the ferromagnetic portion 1 in the compacting direction (axial direction), and .sigma..sub.s is a saturation magnetization of the core 3. To produce an R.R. magnet having a large L, Lm of the ferromagnetic portion 1 is required to be increased. However, Lm cannot be freely increased. Since Bg should be about 0.5 T (tesla) to magnetically orientate the starting powder in the cavity 10 in the radial direction and .sigma..sub.s is usually about 2 T, the value of Lm is limited by the following formula (2): EQU Lm.ltoreq.d.sup.2 /D (2).
With this limitation of Lm, an R.R. magnet having L exceeding the above limitation of Lm has been difficult to be produced in a single compacting operation. Therefore, such an R.R. magnet has been produced by binding a plurality of R.R. magnet parts produced by using a die having a small Lm satisfying the formula (2). However, this method suffers from the defect such as decreasing in the total magnetic flux due to the adhesive layers and/or treating layers present between the R.R. magnet parts and a high production cost due to an increased number of binding steps.
To remove the defect, several methods have been proposed in the prior art. Japanese Patent Laid-Open No. 2-281721 proposes a so-called multi-stage compacting method. In this method, a starting powder in the cavity surrounded by the ferromagnetic portion of the die is compacted into a first green body, which is then shifted downward to the space surrounded by the non-magnetic portion of the die to make the cavity empty. Into the empty cavity, a second amount of the starting powder is charged, compacted to form a second green body on the first green body, and then sifted downward together with the first green body to make the cavity empty again. Thus, the sequential process of charging the powder, compacting the powder and shifting downward the green body is repeated desired times to produce a green body stack which is sintered by a known method to obtain an R.R. magnet having a large L. However, in the proposed method, since each of the compacting steps is carried out under the same pressure, the green bodies have the same density, this resulting in the occurrence of cracking during the sintering process at the binding portion between the green bodies. In addition, since Lm is reduced to create a high orientation magnetic field in the cavity, the proposed method requires an increased number of compacting steps to attain a large L.
Japanese Patent Laid-Open No. 6-13217 proposes another method in which a second amount of starting powder is charged into vacant space in the cavity created by compacting a first amount of starting powder without shifting any green body downward. The sequential steps of charging the starting powder into the vacant space and compacting the starting powder are repeated until the green body stack reaches the desired L. In this method, each compacting step is carried out so that a green body has a density of about 3 g/cm.sup.3, and the density of the green body stack is increased to about 4 g/cm.sup.3 in the final compacting step. Although the proposed method can avoid the cracking occurred in the method proposed by Japanese Patent Laid-Open No. 2-281721, a green body stack having L larger than the axial length of the ferromagnetic portion of the die cannot be produced by the method.
The inventors tried to avoid the cracking in the method of Japanese Patent Laid-Open No. 2-281721 by combining the methods of Japanese Patent Laid-Open Nos. 2-281721 and 6-13217, namely, in the multi-stage compacting steps of Japanese Patent Laid-Open No. 2-281721, the density of the green bodies was regulated within 2 to 3 g/cm.sup.3 and increased to 4 g/cm.sup.3 in the final compacting step as taught by Japanese Patent Laid-Open No. 6-13217. Although the cracking was avoided, the resulting magnet was poor in magnetic properties. Also, although the length Lm was reduced to increase the orientation magnetic field intensity, the magnetic properties of the magnet were not improved corresponding to the increased orientation magnetic field.
Japanese Patent Laid-Open No. 7-161524 teaches that the cracking during the sintering process can be avoided by a binding layer rich in rare earth elements which is present between the green bodies. However, the resulting magnet is poor in the corrosion resistance due to a large amount of the corrosive rare earth elements contained in the binding layer even when the magnet is subjected to a surface treatment for improving the corrosion resistance.